World War II Code Is Broken, Decades After POW Used It

As a prisoner of war, Sub Lieut. John Pryor encrypted information and requests for supplies in letters sent from a German camp to his family in Cornwall.

Plymouth University

Originally published on May 5, 2013 4:04 am

It's been 70 years since the letters of John Pryor were understood in their full meaning. That's because as a British prisoner of war in Nazi Germany, Pryor's letters home to his family also included intricate codes that were recently deciphered for the first time since the 1940s.

Pryor's letters served their purpose in World War II, as Britain's MI9 agents decoded the messages hidden within them — requests for supplies, notes about German activities — before sending them along to Pryor's family in Cornwall.

"There were two types of information buried in these letters," Pryor's son, Stephen, tells Weekend Edition Saturday's Scott Simon. "There is military intelligence going back about munitions dumps, about submarines that have been sunk, and information requests for British Military Intelligence in London to send maps and German currency and German ID, to help them with their escape plans."

After the war, Pryor lived a long life; he died in 2010 at age 91. But he also forgot the intricate code he used to communicate after being taken prisoner at Dunkirk. The letters came under new scrutiny recently after Stephen Pryor, the chancellor of Plymouth University, mentioned them to a military intelligence expert at the school. That led them to team up with a historian and a mathematician. Eventually, they cracked the code.

As an example, Pryor reads a segment of a letter: "I am pleased that I've got the two letters telling me of my cousin's latest event; how happy he must undoubtedly be."

The passage contains coded information about a submarine, the HMS Undine, Pryor says. And the code is far from simple.

"You take the first letter of every word in groups of three," Pryor says. "And then it goes into a three-dimensional matrix, which you have to remember in order to decode and get the sequence of letters to produce the name of the vessel."

The letters passed through German censors, and then through the hands of British agents, before finally reaching John Pryor's family. In cases where POWs sent information in code, intelligence officials "informed the relatives that some letters would read a little strangely," Stephen Pryor says.

The prisoners also used subtle cues, such as including certain words or underlining their signature, to signal to British intelligence that a letter contained coded information.

Stephen Pryor says that after the war, his father mostly kept quiet about the code, and about his wartime experience.

"But I can see now that he was among tens of thousands of other young men who gave up their youth in captivity," he says. "He and his peers took incredible risks, and that has only made me admire him, and all the other men, for their resilience and ingenuity."

You can read a longer passage from one of John Pryor's letters in The Daily Mail, along with its decoded meaning.

At first it reads like any other letter home. My dear mommy and daddy, it begins. Last week I received a short letter from Robert. It was written 64 years ago by John Pryor, who was a British prisoner of war in a German camp, but the letter is not ordinary.

John Pryor put a secret code into his descriptions of everyday life in a prison camp to try to tip British Intelligence about what he could observe even in captivity. A team of researchers at Plymouth University has just helped John Pryor's son decode his father's wartime letters home. Stephen Pryor joins us now from BBC studios in Plymouth. Thanks very much for being with us.

STEPHEN PRYOR: Thank you.

SIMON: What kind of information was your father trying to get out?

PRYOR: There were two types of information buried in these letters. There is military intelligence going back about munitions dumps, about submarines that have been sunk and information requests for British Military Intelligence in London to send maps and German currency and German I.D. to help them with their escape plans.

SIMON: How did he manage that?

PRYOR: Well, it's a complicated code. Can I give you an example...

SIMON: Yes.

PRYOR: ...of a sentence?

SIMON: Please.

PRYOR: This one letter is about the sinking of a submarine called HMS Undine and there's a sentence here that says: I am pleased that I have got the two letters telling me of my cousin's latest event. How happy he must undoubtedly be. That sentence is a spelling of the name of the submarine, HMS Undine.

SIMON: What is it, the first letter of every word?

PRYOR: More than that. You take the first letter of every word in groups of three and then it goes into a three-dimensional matrix which you have to remember in order to decode and get the sequence of letters to produce the name of the vessel.

SIMON: Did the recipients of these letters know what he was doing?

PRYOR: Military intelligence, once they knew that a prisoner had the code and would be writing home, informed the relatives that some letters may read a little strangely, so the relatives would have known that the letter was in code, but they would never have been told what was in the letters.

SIMON: Mr. Pryor, even given what we understand now, the very good reason for some of the stilted language and innocuous observations, what do you learn about your father by rereading these letters?

PRYOR: My father, until the later years of his life, didn't talk much about his war experiences, but I can see now that he was among tens of thousands of other young men who gave up their youth in captivity. He and his peers took incredible risks and that has only made me admire him and all the other men for their resilience and ingenuity.

SIMON: Stephen Pryor, who worked with Plymouth University researchers to crack the code in his father's letters home from a POW camp in Germany. Mr. Pryor, thanks so much.